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Nuclear chemistry

Panning for ununbium

The chemical identification of two atoms of element 112 — scooped from the helium stream they were suspended in using a gold pan — brings the superheavy elements' fabled island of stability into sharper focus.

The superheavy element 112, provisionally named ununbium, is expected to belong to group 12 of the periodic table, which includes the familiar transition metals zinc, cadmium and mercury. As a metal, mercury is already quite unusual. It is volatile — it is liquid at room temperature — and chemically rather inert. A further peculiarity is its ability to form alloys, known as amalgams, with many metals. For that reason, miners traditionally used mercury to recover gold from its ores (Fig. 1a). On page 72 of this issue, Eichler et al.1 turn the tables, using gold to pan for mercury's heavier homologue, element 112. Against all odds, they struck rich. Their haul was a mere two atoms that decayed again within a few seconds, but the consequences are nonetheless far-reaching.

Figure 1
figure1

UNDERWOOD & UNDERWOOD/CORBIS

Element panning, old-school style.

Superheavy elements owe their existence solely to so-called nuclear-shell effects2: certain 'magic' combinations of many protons and neutrons arrange themselves so favourably that they can act against the disruptive forces of the protons' charges. Nuclei with these configurations are, unlike many heavy nuclei, fairly stable against spontaneous fission. The most stable nuclei might survive up to years — an effect comparable to the chemical inertness of the noble gases elsewhere in the periodic table. Such nuclei are thought to inhabit an 'island of stability' conjectured to lie on the horizon of the current periodic table.

Shortly after physicists postulated the existence of superheavy elements in the 1960s, chemists joined in the rush to prospect for them. This not only involved a frantic search for these elusive elements in nature, but also led to the construction of new accelerators and the development of fast online detection techniques3. The euphoria was fuelled by rather exotic predictions: “Are elements 112, 114 and 118 relatively inert gases?” asked the renowned theoretical chemist Kenneth Pitzer in 1975 (ref. 4). Owing to the superheavy atoms' very large nuclear charge, some electrons in them move with velocities close to the speed of light, and must be treated relativistically. This relativistic correction might significantly alter the order of the elements' electronic orbitals, and so severely limit their chemical reactivity. The superheavy elements thus developed into a sort of sanctuary for theoretical chemists: any prediction made there was comparatively safe from experimental verification.

But this situation has been changing. The synthesis of several superheavy nuclei in fusion reactions involving heavy ions has been reported recently5. These spectacular results had, however, been questioned because other groups had failed to reproduce them. Eichler et al.1 have just succeeded — at least in one case.

In an experiment lasting several weeks, the authors, working at the Flerov Laboratory of the Joint Institute for Nuclear Research in Dubna, Russia, bombarded a target of the plutonium isotope 242Pu with an intense beam of calcium (48Ca) ions. Their hope was that two nuclei would occasionally fuse to form the superheavy isotope 287114, after the emission of three neutrons. From previous experiments at the same laboratory, 287114 was known to decay within about half a second by the emission of an α-particle (a helium nucleus, 4He, of two protons and two neutrons) to 283112.

Owing to the relatively long half-life (several seconds) of 283112, this nucleus can be trapped in helium gas and thus swept to a detector consisting of a narrow channel formed of gold-covered silicon detectors. To cover for the eventuality that element 112 is highly volatile (as, for example, the heavy noble gas radon is), the authors cooled the far end of this channel to −180 °C. Under these same conditions, a diffusion-led process would deposit radioactive mercury isotopes on to the gold surface near the warm entrance of the detector; the still-more-volatile radon isotopes, by contrast, would be collected near the cold end.

So, would single atoms of element 112 behave in a manner becoming of their class — in other words, similarly to mercury, as predicted by new, very elaborate theoretical predictions6? Or would they act more exotically — that is, like radon, as speculated by Pitzer4? The former, say Eichler and colleagues, who observed two unique mercury-like signals near the warm end of their detector. First, they saw an α-particle with an energy of 9.5 mega-electronvolts, characteristic of the decay of 283112 to the darmstadtium isotope 279Ds; the signal was followed shortly after by the signal of this isotope's spontaneous fission.

The observations of long-lived α-decay of 283112 confirm that the island of stability is not just a mirage. Indeed, the ground on its neutron-poor shores is seeming ever more solid. As the synthesis of relatively long-lived isotopes of element 114 in reactions of 244Pu and 48Ca has been reported, the next step should be to perform first experiments with this element. After all, element 112 seems to behave chemically quite 'normally'; that is, similarly to its lighter group partner mercury — although it could be that 112 is considerably more inert and volatile. The current experiment was not designed to investigate that, and further observations are needed.

So should we resume searching for superheavy elements in nature? Perhaps: armed with experiments such as those of Eichler and colleagues1, we at least have a much better idea what to look for. Like the authors, we might, against all odds, strike it rich.

References

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Türler, A. Panning for ununbium. Nature 447, 47–49 (2007). https://doi.org/10.1038/447047a

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